U.S. patent number 6,404,481 [Application Number 09/578,573] was granted by the patent office on 2002-06-11 for adaptive lithography membrane masks.
This patent grant is currently assigned to Board of Supervisors of Louisiana State University and Agricultural and Mechanical College, Massachusetts Institute of Technology. Invention is credited to Martin Feldman, Michael H. Lim, Ken-Ichi Murooka, Henry I. Smith.
United States Patent |
6,404,481 |
Feldman , et al. |
June 11, 2002 |
Adaptive lithography membrane masks
Abstract
Techniques are disclosed to compensate for distortions in
lithography by locally heating the membrane in a lithographic mask.
The techniques may be used both to shrink and to expand areas of
the mask locally, in order to adjust for varying magnitudes and
signs of distortion. In one embodiment the correction method
comprises two steps: (1) A send-ahead wafer is exposed and measured
by conventional means to determine the overlay errors at several
points throughout the field. (2) During exposure of subsequent
wafers, calibrated beams of light are focused on the mask. The
heating from the absorbed light produces displacements that
compensate for the overlay errors measured with the send-ahead
wafer. Any source of distortion may be corrected--for example,
distortion appearing on the mask initially, distortion that only
develops on the mask over time, or distortion on the wafer. In
another embodiment, a reference pattern is formed on the membrane
as a means of measuring mask distortion, and the heat input
distribution needed to correct distortion is determined by
subsequent measurements of the reference pattern. In this
alternative embodiment, any source of distortion in the mask may be
corrected.
Inventors: |
Feldman; Martin (Baton Rouge,
LA), Smith; Henry I. (Sudbury, MA), Murooka; Ken-Ichi
(Yokohama, JP), Lim; Michael H. (Cambridge, MA) |
Assignee: |
Board of Supervisors of Louisiana
State University and Agricultural and Mechanical College (Baton
Rouge, LA)
Massachusetts Institute of Technology (Cambridge,
MA)
|
Family
ID: |
24313439 |
Appl.
No.: |
09/578,573 |
Filed: |
May 25, 2000 |
Current U.S.
Class: |
355/52; 355/30;
355/53; 355/67; 355/72; 378/34; 378/35 |
Current CPC
Class: |
G03F
1/20 (20130101); G03F 1/22 (20130101); G03F
7/70433 (20130101); G03F 7/70633 (20130101); G03F
7/70875 (20130101) |
Current International
Class: |
G03F
7/20 (20060101); G03B 027/68 (); G03B 027/52 ();
G03B 027/42 (); G21K 005/00 () |
Field of
Search: |
;378/34,35
;355/30,53,67,72,52 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Aoyama, H. et al., "Magnification correction by changing wafer
temperature in proximity X-ray lithography," J. Vac. Sci. Technol.
B, vol. 17, pp. 3411-3414 (1999). .
Feldman, M., "Thermal Compensation of X-ray Mask Distortions,"
Poster presented at the 43rd International Conference on Electron,
Ion and Photo Beam Technology and Nanofabrication (Marco Island,
Florida, Jun. 1-4, 1999). .
Feldman, M., "Thermal Compensation of X-ray Mask Distortions," J.
Vac. Sci. Technol. B, vol. 17, pp. 3407-3410 (1999). .
Feldman, M. et al., "Thermal Compensation of X-ray Mask
Distortions," Abstract of Poster presented at the 43rd
International Conference on Electron, Ion and Photo Beam Technology
and Nanofabrication (Marco Island, Florida, Jun. 1-4, 1999). .
Feldman, M. et al., "Wafer chuck for magnification correction in
X-ray lithography," J. Vac. Sci. Technol. B, vol. 16, pp. 3476-3479
(1998). .
Fisher, A. et al., "Pattern transfer on mask membranes," J. Vac.
Sci. Technol. B, vol. 16, pp. 3572-3576 (1998). .
Murooka, K. et al., "Membrane-mask distortion correction:
analytical and experimental results," paper to be presented at the
International Conference on Electron, Ion, and Photo Beam
Technology and Nanofabrication (Palm Springs, CA, May 20 to Jun. 2,
2000). .
(unpublished) pending proposal to DARPA BAA 00-4 (prepared Feb. 8,
2000). .
U.S. application No. 09/578,573, Feldman, filed May 25, 2000. .
U.S. application No. 09/578,572, Feldman, filed May 25,
2000..
|
Primary Examiner: Adams; Russell
Assistant Examiner: Brown; Khaled
Attorney, Agent or Firm: Runnels; John H.
Claims
What is claimed:
1. A method to compensate for distortion in lithographic patterning
with a membrane mask, said method comprising the following
sequential steps:
(a) forming a reference pattern on a mask;
(b) forming a masking pattern on the mask;
(c) measuring the reference pattern, and determining from the
changes in the measured reference pattern the distribution of
distortion in the mask following said forming of the masking
pattern; and
(d) illuminating the mask with light, wherein the location and
intensity of said illuminating light differentially heat the mask,
to cause displacements within the mask, wherein the displacements
compensate for the determined distortion.
2. A method as recited in claim 1, additionally comprising the step
of measuring the reference pattern prior to said forming of the
masking pattern; and wherein step (c) of said method comprises
determining changes in the measured reference pattern after said
forming of the masking pattern, as compared to the reference
pattern as measured prior to said forming of the masking
pattern.
3. A method as recited in claim 1, wherein the reference pattern is
formed by interferometric lithography.
4. A method as recited in claim 1, wherein the reference pattern is
formed on the same side of the mask as the masking pattern.
5. A method as recited in claim 4, wherein the reference pattern is
measured with light from the opposite side of the mask from the
masking pattern.
6. A method as recited in claim 1, wherein the reference pattern is
formed on the opposite side of the mask from the masking
pattern.
7. A method as recited in claim 1, wherein each of said measuring
steps comprises measuring with a holographic-phase-shifting
interferometer.
8. A method as recited in claim 1, wherein steps (c) and (d) of
said method are performed while the mask is mounted in a
lithographic apparatus for patterning a wafer.
9. A method as recited in claim 8, wherein steps (c) and (d) of
said method are performed during lithographic patterning of a wafer
through the mask.
10. A method as recited in claim 1, additionally comprising steps
(e) and (f) below:
(e) measuring the reference pattern during said illuminating, and
determining any deviations in the measured reference pattern;
and
(f) adjusting said illuminating so that the resulting differential
heating of the mask causes displacements that further reduce the
determined distortion of the reference pattern.
11. A method as recited in claim 1, wherein said irradiation
comprises X-ray radiation.
12. A method as recited in claim 1, wherein said irradiation
comprises ultraviolet light.
13. A method as recited in claim 1, wherein said irradiation
comprises deep ultraviolet light.
14. A method as recited in claim 1, wherein said irradiation
comprises visible light.
15. A method as recited in claim 1, wherein said irradiation
comprises electrons.
16. A method as recited in claim 1, wherein said irradiation
comprises ions.
17. A method as recited in claim 1, wherein said illuminating
comprises illuminating with a modulated laser beam.
18. A method as recited in claim 1, wherein said illuminating
comprises illuminating with light reflected from a micro-mirror
array.
19. A method as recited in claim 1, wherein said illuminating
comprises illuminating with light through a transparency, wherein
the transmissivity of the transparency varies to cause the location
and intensity of the transmitted light to vary to differentially
heat the mask to compensate for the distortion.
Description
The development of this invention was partially funded by the
Government under contract number N00019-98-K-0110 awarded by the
Defense Advanced Projects Research Agency. The Government has
certain rights in this invention.
This invention pertains to apparatus and methods for compensating
distortions in membrane lithography masks, wafers, or both.
The size of circuit elements used in state-of-the-art integrated
circuits continues to decrease. New lithography techniques will be
needed to continue this reduction to sizes much smaller than those
currently in use. Proximity X-ray lithography is a particularly
promising technique, as it allows the largest exposure field of any
of the contenders for the next generation of integrated circuit
lithography, on the order of 5 cm.times.5 cm in a single exposure.
The large exposure field provides a significant throughput
advantage, but it also makes image placement more critical, to the
point where accurate image placement is widely regarded as a major
factor limiting the use of X-ray lithography in very large scale
integrated ("VLSI") circuits. Overlay errors in proximity X-ray
lithography may arise from several factors, including for example
the following: (1) errors in the pattern writing tool; (2)
distortions in the membrane mask caused, for example, by stresses
in the absorber; and (3) distortions that are already present in
the pattern on the wafer. Much effort has gone into minimizing all
three effects, as well as at least partially compensating them by
adjusting the magnification.
The industry's response to this critical problem has typically been
to design masks that are as rigid as possible, to try to minimize
one potential cause of distortion at its source. While other types
of masks are inherently more rigid, the inherent rigidity of
membrane masks is relatively low. The rigidity of membrane masks
has been increased, for example, by the use of diamond substrates.
Membrane masks are currently required for X-ray lithography, ion
beam lithography, and some types of e-beam lithographies. Although
membrane masks may in principle be used in almost all lithography
techniques, they have generally been considered less desirable. One
approach in a projection electron beam system (the so-called
SCALPEL system) has been to reinforce the required membrane masks
with "grillage" to increase their rigidity.
The type of "membrane mask" used in X-ray lithography comprises a
membrane and an absorber. The membrane is a continuous sheet that
is relatively transparent to the radiation used to expose a resist
on a wafer. The main function of the membrane is to support the
absorber. The absorber, which adheres to the membrane, is
relatively opaque to the radiation. The absorber is patterned to
correspond with the pattern desired in the exposed and developed
resist on the wafer, and need not be continuous since it adheres to
the membrane.
In other lithographies, other types of membrane masks have been
used. For example, the membrane may be opaque to the radiation,
except where holes are placed in the membrane (so-called "stencil"
masks). Alternatively, the absorber may be replaced by a patterned
layer that scatters incident radiation instead of absorbing it.
Generally, the industry has addressed the problem of distortion by
trying to manufacture masks that are as accurate as possible,
considering the ideal to be features positioned on orthogonal,
perfectly linear axes. Much of the cost of mask-patterning tools
lies in the references, metrology, and feedback used to enhance
accurate image placement. However, this approach cannot accommodate
changes in a mask that occur in processing steps subsequent to
resist exposure, nor accommodate changes that occur as a mask ages,
nor match a mask to distortions that may exist on the wafer being
exposed.
A technique called "pattern-specific emulation" has been used to
compensate for distortion in X-ray masks, such as distortion caused
in etching the absorber. In this method a "send ahead" mask is
first made, and is then used to expose a level on a wafer. Pattern
displacements from the desired positions are noted, and are fed
back to the mask-writing tool. A new mask is then written
incorporating these displacements. Although time consuming and
costly, this method did improve overlay. See, e.g., A. Fisher et
al., "Pattern transfer on mask membranes," J. Vac. Sci. Technol. B,
vol. 16, pp. 3572-3576 (1998).
R. L. Engelstad and F. Cerrina of the University of Wisconsin have
considered the displacement of features on a membrane mask by
heated gas jets (private communication).
Magnification correction is considered one of the critical issues
in lithography. In some lithographic techniques, magnification
correction has been accomplished by an adjustment of the exposure
tool. For example, in projection optical lithography it is routine
to adjust the magnification by axial displacement of the reticle
and refocusing in a projection system which is non-telecentric on
the reticle side. Similarly, adjusting the gap in point-source
X-ray lithography changes the magnification. Adjusting
magnification is more difficult in storage-ring X-ray
lithography.
It has also been proposed to correct for magnification errors by
expanding or contracting either the mask or the wafer. Both
mechanical and thermal means have been suggested for correcting
magnification errors.
U.S. Pat. No. 5,155,749 discloses expansion of an X-ray membrane
mask by heating a support ring to facilitate magnification matching
between the mask and the wafer.
A method has been proposed to correct magnification errors by
preheating the wafer, and then vacuum-chucking it so that its size
is "frozen in" by the chucking force when the wafer is cooled back
down. See H. Aoyama et al., "Magnification correction by changing
wafer temperature in proximity X-ray lithography," J. Vac. Sci.
Technol. B, vol. 17, pp. 3411-3414 (1999).
U.S. Pat. No. 5,504,793 discloses a method for applying torque to
an X-ray mask at severallocations around its edge with mechanical
actuators, to stretch or compress the mask membrane to provide
magnification correction.
It has been proposed to mount the wafer on a spherical vacuum
chuck, and to adjust the size of the front surface by changing the
radius of the chuck via application of an internal pressure or
vacuum. See M. Feldman et al., "Wafer chuck for magnification
correction in X-ray lithography," J. Vac. Sci. Technol. B, vol. 16,
pp. 3476-3479 (1998).
A novel method has been discovered, called the adaptive membrane
mask technique, for locally heating the membrane in a lithographic
mask to compensate for distortion in the mask, the wafer, or both.
This technique may be used both to shrink and to expand areas of
the mask, in order to adjust for varying magnitudes and signs of
distortion. The adaptive membrane mask represents a major change in
philosophy, from increasingly costly and difficult "dead reckoning"
methods to one using feedback.
In one embodiment, the correction method comprises two steps: (1) A
send-ahead wafer is exposed and measured by conventional means to
determine the overlay errors at several points throughout the
field. The errors may be the result of distortion in the mask, in
the wafer, or both. (2) During exposure of subsequent wafers,
calibrated beams of light are focused on the mask. The source of
light may be, for example, a modulated laser beam, a halogen lamp,
a capillary lamp, or a cathode ray tube. The light could have
wavelengths from infrared to visible to ultraviolet. The light may
be scanned across the mask, or projected on the mask directly or
through a transparency or liquid crystal array, or produced by
another projection system. The heating from the absorbed light
produces displacements that compensate for the overlay errors
measured with the send-ahead wafer. While heating a portion of a
mask causes it to expand, a portion of a mask may also effectively
be shrunk by heating the areas surrounding it.
In some circumstances, an alternative embodiment may be preferred.
There is a delay inherent in the send-ahead wafer technique, due to
the time required to develop the wafer, to measure its distortions,
and to prepare a suitable transparency (if a transparency is used).
The cost of the "down time" for a lithography exposure tool, for
example one on a synchrotron X-ray source, can be significant. In
such a case, an alternative embodiment may be used to essentially
eliminate such "down time," thereby reducing costs. Prior to using
the exposure tool, distortions in the mask are measured off-line,
and optionally the distortions in the wafer may be measured
off-line as well. Based on the off-line measured distortions, the
required compensations are calculated in advance, so that no time
is lost while using the exposure tool itself
The novel compensation method allows any source of distortion to be
corrected--for example, distortion appearing in the mask as
manufactured, distortion that only develops in the mask over time,
or distortion in the wafer--in the latter case, particularly
systematic distortions that are repeated from one wafer to the
next.
This compensation method is simplified in the particular case of a
scanned exposure using X-rays from an electron storage ring, since
at any given time a correction need only be applied in the
immediate vicinity of the line currently exposed by the X-ray beam.
Two light beams may be used, one forming a line image just above,
and one just below the X-ray beam. FIG. 1 illustrates one
embodiment in accordance with the present invention in a storage
ring beam line. The transparency partially transmits the light, and
is scanned synchronously with the mask and wafer. Preferably, the
lamp is a line source perpendicular to the plane of the paper. For
clarity, only the beam forming a line image below the X-ray beam is
shown; in practice some of the components could be shared by the
light beams both above and below the X-ray beam. Intensity
differences between the two beams cause differential mask expansion
in the vertical direction, compensating vertical distortion, while
intensity variations common to both beams compensate in the
horizontal distortion; see FIGS. 7(a), 7(b), and 7(c). (It is
understood that the designations "horizontal" and "vertical" refer
to a direction parallel to the line being exposed by the radiation,
and a direction perpendicular to that line, respectively.)
The loads produced by localized heating of the mask may be used to
restore the mask to an undistorted condition, or to match an
existing distortion pattern on the wafer. This ability gives
membrane masks a flexibility that rigid masks lack. Instead of
making the mask and the exposure tool as perfect and as fixed as
possible, a task that is rapidly becoming a critical technical
barrier to nanoscale and VLSI lithography, the adaptable mask in an
image placement feedback system in principle allows the image
placement accuracy to be made as good as the ability to measure
it.
In another alternative embodiment, a mask may be fabricated and
handled so that in use it is nearly free of distortion. This
alternative embodiment preferably employs the following sequence of
steps: (1) forming a fiducial grid on the mask prior to the
formation of the masking pattern, preferably by interferometric
lithography, and preferably in or on the membrane; (2) an optional
step of characterizing the fiducial grid prior to formation of the
masking pattern, for example by means of a
holographic-phase-shifting interferometer, such as that shown for
example in FIG. 8, and as otherwise described in M. Lim et al., J.
Vac. Sci. Technol. B, vol. 17, pp. 2703-2706 (1999); and in K.
Murooka et al., "Membrane-mask distortion correction: analytical
and experimental results," paper to be presented at the
International Conference on Electron, Ion, and Photo Beam
Technology and Nanofabrication (Palm Springs, Calif., May 30 to
Jun. 2, 2000); (3) forming the masking pattern on the membrane, for
example an X-ray absorber pattern in the case of an X-ray mask; (4)
measuring the fiducial grid again, for example with a
holographic-phase-shifting interferometer, to determine the
distribution of distortion associated with, or caused by, the
masking pattern; (5) calculating the stress distribution that
produced the distortion distribution determined in step (4), and
calculating from the stress distribution a heat-input distribution
to compensate or correct the distortion distribution; and (6)
applying the calculated heat input distribution to the mask.
Two techniques, an analytical method and a finite elements method,
for calculating a heat-input distribution to compensate or correct
a measured distortion distribution, are disclosed in (unpublished)
pending proposal to DARPA BAA 00-4 (prepared Feb. 8, 2000), a
complete copy of which is being submitted with this application as
originally filed in the United States Patent and Trademark Office,
and the complete disclosure of which is hereby incorporated by
reference.
An advantage of using such a fiducial grid and the associated
interferometric means of measuring distortion of the fiducial grid
is that these measurements may be made while the mask is located in
an exposure apparatus in preparation for exposure. Moreover, the
measurement can be performed while the heat input is being applied,
and hence during the actual exposure, allowing active feedback to
the heat input.
FIG. 8 depicts a holographic phase-shifting interferometer that may
be used in making these measurements. As compared to an
interferometric lithography system such as is known in the art, the
holographic phase-shifting interferometer has three primary
modifications: (1) A fluorescent or non-fluorescent screen is
placed in front of one pinhole to capture the interference pattern
between the reflected and back-diffracted beams; (2) A
piezoelectric transducer pushes the beam-splitter, which in turn
causes a phase shift in one of the arms; and (3) A CCD camera (not
shown) is used to record the fringe patterns.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates one embodiment in accordance with the present
invention in a storage ring beam line.
FIG. 2 illustrates schematically the experimental apparatus used
for measurements of thermal displacements on X-ray masks.
FIG. 3 illustrates in-plane thermal displacements observed at the
center of the boron nitride mask as a function of the location.
FIG. 4 depicts observed peak-to-peak displacements measured along
the diameter of the boron nitride mask.
FIG. 5 shows the measured dependence of displacement on the gap for
a silicon membrane mask.
FIG. 6 shows the measured dependence of displacement on the gap for
a silicon carbide membrane mask.
FIGS. 7(a), 7(b), and 7(c) illustrate, respectively, light beams in
the vicinity of an X-ray beam; intensity differences between the
two light beams causing differential mask expansion in the vertical
direction, compensating vertical distortion; and intensity
variations common to both light beams compensating horizontal
distortion.
FIG. 8 depicts a holographic phase-shifting interferometer.
The light focused on the mask deposits heat, resulting in a
temperature distribution T(X,Y) in the mask membrane, and hence
in-plane displacements in that membrane. The magnitude of these
displacements depends on the location within the mask, the
locations at which the heat is deposited, the gap between the mask
and the wafer, and the material properties of the mask membrane,
such as its coefficient of thermal expansion and its Young's
modulus.
Under the approximation of steady state conditions, the input heat
is exactly balanced by heat lost from the mask. Neglecting heat
flow in the plane of the mask, and assuming that the wafer is at a
constant temperature, much of the heat flow from the mask results
from thermal conduction across the gap, at a rate inversely
proportional to the gap dimension. We have ##EQU1##
where g is the distance of the gap, .delta.T=.delta.T(X,Y) is the
difference in temperature between the wafer and the mask, C.sub.1
is proportional to the heat lost through the gap, and C.sub.2 is
proportional to the heat lost by all other mechanisms. Since the
in-plane displacement is directly proportional to the magnitude of
the temperature distribution within the mask, if .delta.T=0 for at
least one place on the mask, we may write ##EQU2##
where p is the power (in the form of heat) deposited on the mask by
light.
Experimentally-determined values of C.sub.1 and C.sub.2 are given
in Table I for three particular masks: an AT&T-Bell Labs boron
nitride mask that was 15 years old at the time of the measurements,
and more modern IBM silicon and silicon carbide masks.
TABLE I Properties of Masks Tested AT & T-Bell Manufacturer
Labs IBM IBM Membrane Boron nitride Silicon Silicon carbide
Membrane Thickness 8 .mu.m 2 .mu.m 2 .mu.m Absorber Gold Gold
Tantalum silicide Absorber Thickness 0.7 .mu.m NA 0.6 .mu.m
Absorber Coverage 100% 70% 10% Membrane Size 72 mm Diameter 40
.times. 40 mm 26 .times. 33 mm Total Mask Thickness 5.7 mm 6.4 mm
7.6 mm Small Gap Sensitivity .+-.5 .+-.7 .+-.11 (nm/W/.mu.m) Small
Gap Sensitivity 0.7 1.75 3.3 (ppm/W/10 .mu.m air gap) C.sub.1 (W)
186 143 143 C.sub.2 (W/.mu.m) 0.47 0.62 0.47
Measurements of thermal displacements on X-ray masks were performed
on a microscope with a 50.times., 0.55 NA, ultra-long working
distance objective (Olympus ULWD MS Plan 50.times. microscope
objective, Olympus America Inc., Melville, N.Y.). FIG. 2
illustrates schematically the experimental apparatus used. The
microscope was equipped with a video camera that had been modified
to provide approximately twice the nominal magnification.
Measurements were mad e by viewing the video signal from the camera
directly with an oscilloscope; the video sweep speed was calibrated
at 1.23 .mu.m per .mu.sec with a stage micrometer (Model F36121,
Edmund Scientific Company, Barrington, N.J.). The observed video
rise time was equivalent to approximately 0.6 .mu.m, consistent
with the nearly diffraction-limited spatial resolution previously
observed for the microscope objective.
Masks were viewed either "face up" on an optical flat to provide a
relatively large gap to the nearest cooling surface, or "face down"
on the optical flat to provide a relatively small gap. In the
latter case shims were used to adjust the gap, which was measured
by focusing the microscope alternately on the mask and on scratches
on the optical flat.
The heating light beam was obtained from a 35 mm projector that had
been modified by removing its condensing optics. The projection
lens was operated at nearly unit magnification, at about
.function./7, to form a 1 cm diameter image of the projection lamp.
The entire projector was mounted on an X-Y stage, pointing downward
at a 30.degree. angle, so that the focused spot could be positioned
throughout the mask surface, including the area directly under the
objective. The area illuminated on the mask was a 10.times.20 mm
ellipse; measurements were made along the direction of the 10 mm
axis.
Mask displacements were generally measured at reduced voltage, with
about 0.31 Watts in the focused light spot. The displacements were
observed to be proportional to power, and to be independent of the
lamp's color temperature, up to full voltage, or about 0.45 Watts
in the focused spot. No out-of-plane displacements were observed,
except at the largest gaps.
In-plane thermal displacement measurements were made on a boron
nitride membrane X-ray mask, manufactured by AT&T-Bell Labs in
1984. FIG. 3 illustrates the displacements observed at the center
of the boron nitride mask as a function of the location of the 0.31
Watt heat input. The mask membrane was a circle 72 mm in diameter.
The mask was placed face-up, to obtain an effective gap equal to
the 5.7 mm thickness of the mask rim. Measurements were made at the
center of the mask while the focused light spot was moved across a
diameter. A maximum "peak-to-peak" displacement of approximately 3
.mu.m was observed as the focused spot traversed from one side of
the microscope objective to the other. The relatively large
displacements shown in FIG. 3 were attributed primarily to the
large gap between the mask and the optical flat.
The peak-to-peak displacement was also measured as a function of
the position of the focused light spot across a diameter of the
mask. FIG. 4 depicts observed peak-to-peak displacements measured
along the diameter of the boron nitride mask, when irradiated by
0.31 Watts of heat near the point of observation. The mask was
again measured with a 5.7 mm gap to the closest surface. The mask
contained 14 test patterns, arranged 5 mm apart from one another in
a linear array, with the first and last patterns spaced 2.5 and 4.5
mm, respectively, from the edge of the membrane. Although the
displacement was constrained to fall to zero at the edge of the
membrane, it rose rapidly away from the edge, and was nearly
constant over a large portion of the mask. Near the edge of the
membrane, most of the peak-to-peak displacement arose from light
that fell between the microscope objective and the edge. This was
true even when only a portion of the light fell on the membrane.
The results shown in FIG. 4 show that even a slightly oversized
membrane should provide adequate correction capability at the edge
of the field.
Since future X-ray exposures are likely to be performed at gaps
less than 10 .mu.m, the dependence of the displacement on the gap
is important. FIGS. 5 and 6 show the measured gap dependence for an
IBM silicon membrane mask and an IBM silicon carbide "Talon")
membrane mask, respectively. FIG. 5 illustrates the measured
peak-to-peak displacement for the IBM silicon membrane mask as a
function of the gap. The solid curve is two times equation (2),
with C.sub.1 =143 W, C.sub.2 =0.62 W/.mu.m, and p=0.31 W. FIG. 6
illustrates the measured peak-to-peak displacement for the IBM
silicon carbide membrane mask as a function of the gap. The solid
curve is two times equation (2), with C.sub.1 =143 W, C.sub.2 =0.47
W/.mu.m, and p=0.31 W. Equation (2) provided a reasonable fit to
the observed measurements for both masks, as well as for the boron
nitride mask, except for the smallest gaps in the case of the
silicon carbide mask, where Equation (2) underestimated the
measured displacements by .about.35%. It was interesting that a
given heat input produced the largest displacements at small gaps
on this particular mask.
At gaps of several millimeters, obtained with the masks "face up,"
all three masks were observed to approach thermal equilibrium with
time constants on the order of one second. The theory behind
Equation (2) predicted that the time constant should be
proportional to the displacement. As expected, the time constant
was too short to be observed at the smallest gaps used in the
experiments reported here.
These preliminary results demonstrated that introducing heat with a
focused light beam can produce significant in-plane displacements
in X-ray masks. This effect may be exploited with a send-ahead
wafer to compensate for virtually any distortions that do not vary
too rapidly. The magnitude of the effect observed in the initial
experiments with a silicon carbide mask, about .+-.110 nm, or 3.3
ppm, per Watt for a 10 .mu.m gap in air, was well within the range
needed to provide useful corrections over most of the exposure
field; as were the comparable magnitudes for boron nitride and
silicon masks, .+-.50 nm per Watt and .+-.70 nm per Watt,
respectively. To further enhance the compensation that may be
achieved with this technique, the initial fabrication of the masks
may be optimized accordingly: for example, a somewhat oversized
membrane permits corrections over the entire field, and also allows
both positive and negative magnification corrections. As another
example, one could deposit on an X-ray membrane mask a low Z (low
atomic mass) material or composite that: (1) comprises a substance
that is highly absorbing optically, such as carbon black or another
black substance, and (2) has a large coefficient of thermal
expansion, and (3) does not degrade rapidly when exposed to X-rays.
Such a coating would improve the mask's sensitivity to the novel
compensation technique, without substantially altering the mask's
X-ray characteristics.
Preliminary theoretical calculations using finite-element analysis
have been used to model the thermal deformations in a
square-membrane mask. One result of the model, which agreed with
the experimental data reported above, was that the sensitivity of
the mask to a localized thermal load was nearly constant over most
of the surface of the mask, changing substantially only near the
edges of the mask. This conclusion is important not only because it
demonstrates nearly constant sensitivity, but also because it
demonstrates the ability to correct magnification errors with only
a slightly oversized membrane. Magnification correction is thus a
special case of the general correction technique of the present
invention.
Although this correction technique was originally developed for use
with X-ray lithography, it is not limited to X-ray lithography, but
may also be applied to lithography at other wavelengths and with
particles other than photons, for example, visible wavelengths,
ultraviolet, deep ultraviolet, electron beams, or ion beams.
Membrane masks have previously not been widely used in photon
lithographies other than X-ray lithography, because of the higher
distortion that has typically been seen as compared to that for
more rigid masks. But the novel technique for correcting distortion
will allow membrane masks to be more widely used in lithographies
where it has previously not found widespread application. As just
one of many examples, the novel adaptive mask technique may be used
to correct for variations between cameras in multi-level
lithography.
Other sources of heat, e.g., the X-ray beam itself, may also
introduce some in-plane displacements, which can also be
compensated by appropriately placed light beams carrying comparable
power.
The initial experiments reported here were conducted in air.
Exposure in a helium environment, as is typical in X-ray
lithography, would reduce both wanted and unwanted heat
displacements as compared to those in air, since helium has a
higher thermal conductivity than does air.
Miscellaneous
The complete disclosures of all references cited in this
specification are hereby incorporated by reference. In the event of
an otherwise irreconcilable conflict, however, the present
specification shall control. Also incorporated by reference are the
entire texts of the following references, none of which is prior
art to the present invention: M. Feldman, "Thermal Compensation of
x-ray Mask Distortions," Poster presented at the 43rd International
Conference on Electron, Ion and Photo Beam Technology and
Nanofabrication (Marco Island, Fla., Jun. 1-4, 1999); M. Feldman et
al., "Thermal Compensation of X-ray Mask Distortions," Abstract of
Poster presented at the 43rd International Conference on Electron,
Ion and Photo Beam Technology and Nanofabrication (Marco Island,
Fla., Jun. 1-4, 1999); K. Murooka et al., "Membrane-mask distortion
correction: analytical and experimental results," paper to be
presented at the International Conference on Electron, Ion, and
Photo Beam Technology and Nanofabrication (Palm Springs, Calif.,
May 30 to Jun. 2, 2000); M. Feldman, "Thermal Compensation of X-ray
Mask Distortions," J. Vac. Sci. Technol. B, vol. 17, pp. 3407-3410
(1999); and (unpublished) pending proposal to DARPA BAA 00-4
(prepared Feb. 8, 2000).
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